tive head impacts is limited by methodological weaknesses. For example, helmet-based head impact recording devices are typically set to record only impact forces over a minimum threshold (e.g., 10 g of linear acceleration; see Duma et al., 2005) and, therefore, do not record all impacts to the head. Although recent advances in technical, statistical, and clinical knowledge have helped to improve research on repetitive head impacts, earlier findings have to be viewed in the context of history: Their importance lies more in their groundbreaking attempts to quantify relevant variables and not necessarily in their specific findings.

Findings from Soccer Studies

In soccer, athletes experience repetitive head impacts from using their heads to strike the ball for passing and shooting. Older research involving amateur and professional soccer players indicated an association between cumulative heading and neuropsychological impairments (see, for example, Matser et al., 1998, 1999, 2001; Sortland and Tysvaer, 1989; Tysvaer and Lochen, 1991). One study of 37 former professional soccer players found mild to severe deficits in the areas of attention, concentration, memory, and judgment in 81 percent of the players. The authors speculated that this finding could be indicative of permanent organic brain damage resulting from repeated traumas from heading the ball (Tysvaer and Lochen, 1991). In another study involving 53 active professional soccer players, impairments in memory, planning, and visuo-perceptual tasks were observed and compared with those in non-contact-sport athlete controls. Among the soccer players, performance on these tasks was inversely related to the frequency of heading the ball (Matser et al., 1998). Computed tomography scans of 33 former professional soccer players identified central brain atrophy in one-third of study participants, although scans were only visually inspected, and there were no baseline or control comparisons (Sortland and Tysvaer, 1989).

Several other studies, including more recent ones, involving youth soccer players have found no effect of heading on neurocognitive performance (Broglio and Guskiewicz, 2001; Guskiewicz et al., 2002; Kaminski et al., 2007, 2008; Kontos et al., 2011; Stephens et al., 2010; Straume-Naesheim et al., 2005). For example, Guskiewicz and colleagues (2002) found no differences in neurocognitive function or Scholastic Aptitude Test scores between collegiate soccer players (n=91) and groups of athletes from other contact and non-contact sports (n=96) or between the collegiate soccer players and non-athletic controls (n=53), suggesting that soccer players are not differentially affected by soccer playing (and, by extension, heading).

Furthermore, studies that have directly assessed changes in cognition related to heading a soccer ball have failed to establish any relationship be-

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